Do I become a Portuguese tax resident as soon as I get the D8 residence permit?

Two different authorities, two different questions. AIMA decides whether you may live in Portugal. The Autoridade Tributaria decides whether Portugal taxes your worldwide income, and it never looks at your permit to answer that.

The two tests that actually decide it

You are a Portuguese tax resident if either of these is true in a given year.

1. You spend more than 183 days, consecutive or not, in Portugal during any 12 month period that begins or ends in that fiscal year. A day counts when it includes a night spent in Portugal, complete or partial (article 16(2) of the CIRS), so a day trip with no overnight stay does not count.
2. On any day of that period you keep a dwelling in Portugal in conditions that suggest you intend to hold and occupy it as your habitual home.

Test two is the one that catches D8 holders. The consulate file normally includes accommodation for the whole stay, and in practice that means a twelve month registered lease. A lease of that kind, with your family in it, is exactly the pattern the habitual residence test describes.

Why the permit still matters indirectly

Residency runs from your first day of stay, not from the date printed on the card, so the AIMA backlog does not delay your tax clock. Your NIF address is the other signal: it is how the tax authority records where you live, so switching it to a Portuguese address is a statement in itself. Do that when the move is real, not while you are still scouting.

Getting it wrong is expensive

Assuming a short stay keeps you outside the system while you hold a Lisbon flat is the common mistake. If the tax authority later treats you as resident for those years, it can assess back tax, interest and penalties on worldwide income.

Run the numbers in the Portugal tax estimator, then read the Portugal income tax and tax residency guide for the full framework. This page describes the rules, not your file: a cross border tax adviser should confirm your own dates before you file anything.